Organic electronic and optoelectronic devices such as photovoltaic (PV) devices require deposition of at least one functional film of organic molecules. The organic films can be grown on a substrate through many techniques, for example, vacuum thermal evaporation (VTE), organic vapor phase deposition (OVPD), chemical vapor deposition (CVD), etc. In a VTE process, a powder of organic species is thermally evaporated within a vacuum chamber and a fraction of the evaporated molecules condense on the substrate. In an OVPD process, an organic powder is also evaporated, but in a stream of hot inert carrier gas that transports the organic molecules to a substrate on which the evaporated organic molecules condense.
These techniques for deposition of organic films require a way to reproducibly control growth rate and thin film thickness in real time and in-situ. In the event of a co-deposition of two or more organic species, a system to precisely monitor and control material mixing is also desirable.
Quartz crystal monitoring (QCM) has been generally used to monitor the mass of material deposited on the substrate by measuring the mass of material deposited on the surface of a quartz crystal placed in a close proximity to the substrate. In a VTE process, a precise reading of the deposited mass by QCM can be achieved by means of frequent replacement and recalibration. However, the control over co-deposition of at least two organic molecules in a VTE process requires the use of several QCMs in parallel, and the control remains tedious and imprecise. In an OVPD process, the use of QCM is more delicate and impractical because the frequency shift of a quartz crystal, which is necessary for signal generation, depends not only on the mass deposited but also on temperature and viscosity of the surrounding medium. Therefore, a better technique for monitoring photoactive organic film deposition processing parameters is highly desirable, particularly in an OVPD process.